1. Field of the Invention
The present invention relates to a power source circuit of the switching stabilizing power source type, which is adaptable for portable television sets, video tape recorders, telephone sets, and the like. Further, the invention relates to an electronic device using such a power source circuit.
2. Discussion of the Related Art
In electronic devices, such as portable TV sets, video tape recorders, and telephone sets, it is a common practice to use a power source circuit of the switching stabilizing power source type, in order to secure a stable operation thereof.
FIG. 5 is a circuit diagram showing an arrangement of a peripheral circuit of a power source of a portable video tape recorder (VTR) 1.
The VTR 1 is made up of a power source circuit 10 of the switching stabilizing power source type, a video signal processor circuit 11 for processing an input video signal, and a monitor control circuit 12 for controlling a monitor screen. The video signal processor circuit 11 and the monitor control circuit 12 are supplied with electric power from the power source circuit 10, through power lines La and Lb.
The power source circuit 10 is composed of a triangle wave oscillator 21 for generating a triangle wave voltage, pulse width control circuits 23a and 23b provided in association with the power lines La and Lb, switching transistors 24a and 24b, and smoothing circuits 25a and 25b. The pulse width control circuit 23a includes two comparators 26a and 27a, and the pulse width control circuit 23b also includes two comparators 26b and 27b.
The arrangement of a circuit associated with the power line La of the power source circuit 10, and the operation of the circuit will be described.
The comparator 26a of the pulse width control circuit 23a compares an output voltage of the smoothing circuit 25a with a reference voltage of a reference power source 28. The comparator 27a compares an output voltage of the comparator 26a with a triangle wave voltage outputted from the triangle wave oscillator 21. The width of a pulse signal is controlled by the result of the comparison, and the width-controlled pulse signal is applied to the switching transistor 24a. The smoothing circuit 25a smoothes an output signal of the switching transistor 24a when it is switched. The thus stabilized power source voltage is supplied to the power line La.
The arrangement of the circuit associated with the power line La and its operation that have been described above, are correspondingly applied to those of a circuit associated with the power line Lb. In this way, the stabilized power source voltage is applied to the video signal processor circuit 11 and the monitor control circuit 12.
In the power source circuit 10 of the switching stabilizing power source type, the smoothing circuits 25a and 25b imperfectly smooth the output signals of the switching transistors 24a and 24b, respectively. As a result, noise synchronized with the switching operations of the switching transistors 24a and 24b are contained in the power source voltages of the smoothing circuits 25a and 25b, respectively. The noise enters into the video signal processor circuit 11 and the monitor control circuit 12. This results in reduction of the S/N ratio, and increase of residual noise, and eventually deteriorates the characteristics of the VTR.
For the operation of the video signal processor circuit 11, a reference clock signal is a clock signal outputted from an oscillator 29 oscillating at 3.58 MHz. The reference clock signal is not synchronized with the noise that is synchronized with the switching operation of the transistor. Because of this, a beat noise is frequently generated.